Which of the following is NOT a state function or state variable in a thermodynamic system?

The correct answer is: (b) Work
Explanation
In thermodynamics, properties are classified based on whether they depend on the history of the system or just its current condition.
· State Functions (or State Variables): These depend only on the initial and final states of the system, not on how the change was accomplished. If you know the current state (e.g., Temperature, Pressure), the value is fixed regardless of how the system got there.
· Path Functions: These depend on the specific path or process taken to go from the initial state to the final state.
Work (w) is a path function because the amount of work done depends on the specific process (e.g., an adiabatic process vs. an isothermal process) used to change the system, even if the starting and ending points are identical.
Information Booster:
The distinction is often expressed mathematically:
· State Functions have exact differentials. The net change of a state function in a closed cycle is always zero.
· Path Functions have inexact differentials. The net work done in a cycle is not zero; it is equal to the area enclosed by the path on a pressure-volume graph.
Additional Knowledge (Why other options are State Functions)
· Enthalpy (H): Defined as H = U + PV. Since Internal Energy (U), Pressure (P), and Volume (V) are all state functions, their combination (Enthalpy) is also a state function.
· Pressure (P): This is a fundamental thermodynamic coordinate. The pressure of a gas in a container is a fixed value at a specific moment, regardless of whether the gas was previously compressed or heated to reach that state.
· Internal Energy (U): The First Law of Thermodynamics states that the change in Internal Energy (Delta U) equals heat (q) plus work (w). While heat and work are path functions, their sum (Delta U) is constant between two specific states.